1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to thin walled turbine airfoils with cooling circuits.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine such as an industrial gas turbine engine, a turbine section includes a plurality of rotor blades that react with the hot gas flow passing through the turbine to produce mechanical work by rotating the rotor shaft. In an industrial gas turbine, four stages of rotor blades and stator vanes are used to extract the energy from the flow. As the inlet temperature to the turbine increases, the size of the fourth stage rotor blade also increases because the flow into the fourth stage has higher energy than previous lower temperature engines. These fourth stage rotor blades can be over 30 inches from platform to blade tip, and also have very large taper and twist in order to react with the flow.
With the higher gas flow temperature exposed to the fourth stage blade, internal air cooling is required in order to increase the life of the rotor blade. However, prior art methods of casting turbine blades having internal cooling circuits are not practical with these larger blades. Radial holes cannot be drilled into the blade because of the large amount of twist from the root to the tip. A straight hole cannot be placed within the blade. These large twist blades have large cross sectional areas in the lower span but have thin cross sectional areas in the upper span. Thus, the rotor blade in the upper span is very thin and thus not acceptable to casting processes of the prior art. Also, ceramic cores used for investment casting of these blades cannot be used in these long and highly twisted blades because the ceramic core would also have a long length with high twist. This produces a very brittle core which would un-twist when hanging within the mold used to cast the blade with the internal cooling passages. Core ties would break and result in improper positioning of the core within the mold. Defective blades would be cast that would also increase the overall cost of manufacturing the usable rotor blades. Therefore, there is a need in the art for producing a long rotor blade with thin airfoil walls with a cooling circuit to provide cooling for the blade.
It is an object of the present invention to provide a thin walled turbine airfoil with an internal cooling air circuit to provide cooling for the airfoil.